Spectral simplification in vibrational spectroscopy using doubly vibrationally enhanced infrared four wave mixing

A new form of vibrational spectroscopy has recently been demonstrated using three coherent sources to generate nonlinear four wave mixing (FWM). It is an optical analogue of 2D NMR. The four transitons that occur in FWM include combinations of infrared absorption and Raman transitions and result in doubly vibrationally enhanced (DOVE) four wave mixing (FWM). In this paper, we use acetonitrile in different mixtures as a model system to demonstrate the spectral selectivity that allows DOVE methods to remove the spectral congestion from vibrational spectra of complex mixtures and to discriminate against a strong water background. The selectivity results from two multiplicative vibrational enhancements and the intra- and intermolecular interactions that cause mode coupling. Since DOVE features cannot occur in the absence of made coupling, these methods isolate the spectral features that are associated with interactions. This method promises to have important applications for probing complex biological, chemical, and environmental samples.